1. Field of the Invention
The present invention relates to a tunneling magnetoresistive storage unit (hereinafter referred to as the TMR unit) having a magnetic substance structure.
2. Description of the Background Art
An MRAM is a memory device which utilizes a phenomenon that the resistance to an electric current flowing through a magnetic substance structure varies depending on the direction of electron spin (magnetization direction) in the magnetic substance. TMR units are storage units conventionally used for storing information. Each cell of a magnetic memory, capable of storing one bit of information, includes one each TMR unit and metal-oxide-semiconductor (MOS) transistor.
FIG. 10 is a cross-sectional diagram showing the construction of a conventional MRAM device. In this Figure, designated by the numeral 100 is a TMR unit having a sandwich structure with a thin insulator layer 102 sandwiched between a first magnetic substance layer 101 and a second magnetic substance layer 103. Designated by the numeral 150 is a semiconductor substrate (hereinafter referred to simply as the substrate) on which an access transistor, which is a MOS transistor, is formed, and designated by the numeral 155 are source/drain regions of the access transistor. Designated by the numeral 160 is a readout word line which serves as a gate electrode of the access transistor and designated by the numeral 165 is its write word line. Designated by the numeral 170 is an electrode section for connecting one of the source/drain regions 155 to the first magnetic substance layer 101, designated by the numeral 175 is an interlayer insulator stacked between the individual layers, and designated by the numeral 180 is a bit line. In this structure, the first magnetic substance layer 101 forms a free-spin layer in which the direction of electron spin is not fixed but variable while the second magnetic substance layer 103 forms a fixed-spin layer in which the direction of electron spin is fixed to a specific direction. Since the sandwich structure of the TMR unit 100 has a rectangular shape in top view that is elongate in the direction of the bit line 180, the spin direction in the first magnetic substance layer 101 could easily become parallel to the direction of the bit line 180 (bit line direction). The spin direction in the second magnetic substance layer 103 is fixed to this bit line direction.
Storage (writing) of data into the TMR unit 100 of the aforementioned conventional MRAM device is performed by producing flows of electric currents through the bit line 180 and the write word line 165 and determining the spin direction in the first magnetic substance layer 101 which forms a free-spin layer with the aid of magnetic fields generated by the electric currents, as shown in FIG. 11. Specifically, a binary xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d is written in the TMR unit 100 depending on whether the spin direction in the first magnetic substance layer 101 is the same (parallel) as or opposite (antiparallel) to that in the second magnetic substance layer 103. This data write operation requires at least a specific magnetic field strength to perform. In addition, the data write operation is characterized in that it is carried out in one memory cell where the corresponding bit line 180 and the corresponding write word line 165 intersect each other.
On the other hand, the data stored in the TMR unit 100 is read by applying a voltage across the first magnetic substance layer 101 and the second magnetic substance layer 103 and a voltage to the readout word line 160 to turn on the access transistor and then measuring an electric current flowing into the access transistor. The amount of this electric current is large when the spin direction in the first magnetic substance layer 101 is the same as that in the second magnetic substance layer 103, whereas the amount of the electric current is small when the spin direction in the first magnetic substance layer 101 is opposite to that in the second magnetic substance layer 103. This property is used in the execution of data read operation. Specifically, the data in the TMR unit 100 is read by varying the electrical resistance between the first magnetic substance layer 101 and the second magnetic substance layer 103, turning on the access transistor, and judging the amount of the electric current flowing from the bit line 180 into the access transistor.
In the structure of the conventional MRAM device described above, it is essential that write word lines and readout word lines be separately arranged and it is difficult to reduce the width of the word lines and bit lines due to the need to form a TMR unit in each of their intersecting areas. It is also difficult to reduce the area occupied by each TMR unit as it is formed by stacking platelike magnetic substance layers with an insulator layer sandwiched in between. In addition, when the TMR unit is miniaturized, it requires a greater magnetic field strength, and accordingly a larger writing current, for reversing the spin direction. Due to these properties, the aforementioned structure of the conventional MRAM device is unsuited for achieving miniaturization and a higher level of integration.
This invention is intended to provide a solution to the aforementioned problems of the prior art. Specifically, it is an object of the invention to provide a structure of an TMR unit suited for achieving miniaturization and a higher level of integration. It is another object of the invention to provide a miniaturized MRAM device featuring a higher level of integration by employing such TMR units.
In a first principal form of the invention, a tunneling magnetoresistive storage unit includes a hollow cylinder-shaped first magnetic substance element whose magnetization direction is variable, the first magnetic substance element having one open end, a columnlike second magnetic substance element whose magnetization direction is fixed to one circumferential direction, the second magnetic substance element being formed inside the cylinder-shaped first magnetic substance element, and an insulator layer located between the first and second magnetic substance elements. A tunneling current is flowed between the first and second magnetic substance elements to produce a rotating magnetic field for setting the magnetization direction of the first magnetic substance element to one of its circumferential directions, wherein magnetoresistance due to a change in the magnetization direction of the first magnetic substance element with respect to the magnetization direction of the second magnetic substance element is used to represent binary data.
This construction makes it possible to reduce the amount of electric current required for performing data write operation, decrease the area occupied by the tunneling magnetoresistive storage unit, and achieve miniaturization and a higher level of integration.
In a second principal form of the invention, a tunneling magnetoresistive storage unit includes a first magnetic substance element whose magnetization direction is variable, a second magnetic substance element whose magnetization direction is fixed to one circumferential direction and an insulator layer located between the first and second magnetic substance elements. The first magnetic substance element is formed into a one-piece structure including a columnlike portion and a cylindrical portion which surrounds the columnlike portion and has one open end. The second magnetic substance element is formed into a hollow cylindrical shape having one open end and is located between the columnlike portion and the cylindrical portion of the first magnetic substance element. In the tunneling magnetoresistive storage unit thus constructed, a tunneling current is flowed between the first and second magnetic substance elements to produce a rotating magnetic field for setting the magnetization direction of the first magnetic substance element to one of its circumferential directions, wherein magnetoresistance due to a change in the magnetization direction of the first magnetic substance element with respect to the magnetization direction of the second magnetic substance element is used to represent binary data.
This construction makes it possible to reduce the amount of electric current required for performing the data write operation and improve the reliability of data readout. The construction also serves to decrease the area occupied by the tunneling magnetoresistive storage unit, and achieve miniaturization and a higher level of integration.
In a third principal form of the invention, a tunneling magnetoresistive storage unit includes a hollow cylinder-shaped magnetic substance element whose magnetization direction is variable, the magnetic substance element opening at both ends, a columnlike conductor element passing through the cylinder-shaped magnetic substance element, and an insulator layer located between the magnetic substance element and the conductor element. An electric current is flowed through the conductor element to produce a rotating magnetic field for setting the magnetization direction of the magnetic substance element to one of its circumferential directions, and the magnetization direction of the magnetic substance element is detected from the amount of electric current which flows through the conductor element.
This construction also makes it possible to reduce the amount of electric current required for performing the data write operation, decrease the area occupied by the tunneling magnetoresistive storage unit, and achieve miniaturization and a higher level of integration.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.